Inflammatory bowel diseases (IBDs) including ulcerative colitis and Crohn's disease, are complex diseases that are thought to result from over activation of the immune system directed at luminal antigens of the gastrointestinal tract (12). In the early 1940's it was observed that sulfasalazine, formed by the chemical union of the antibiotic sulfapyridine and 5-aminosalicylic acid (5-ASA; also referred to as mesalamine) by an azo bond, had a beneficial effect in patients with colitis (29). Subsequent clinical studies over the next two decades established that sulfasalazine had efficacy in the treatment of inflammatory bowel disease (30, 31). Additional studies were directed to determine the chemical kinetics of sulfasalazine when administered orally and to determine mechanisms of action (32-34). Approximate 75% of sulfasalazine reaches the colon unchanged. Within the colon the azo bond is split by bacterial enzyme action into metabolites, 5-ASA and sulfapyridine. Following azo bond reduction, most of the sulfapyridine is absorbed from the colon whereas only 20% of 5-ASA is absorbed. The majority of 5-ASA remains in the colon and is recovered in the feces primarily as free 5-ASA.
Postulated mechanisms for the presumed beneficial action of sulfasalazine in the treatment of colitis initially included inhibition of prostaglandin synthesis and inhibition of the lipoxygenase pathway in inflammatory cells such as neutrophils (35,36). Ensuing investigations have established additional therapeutic mechanisms whereby 5-ASA promotes healing and reduces inflammation in IBD (2, 37, 38). These include: free radical scavengers, inhibit T-cell proliferation, inhibit presentation of antigen to T-cells, inhibit adhesion of macrophages and granulocytes, decrease production of interleukins (ILs) and down regulation of the transcription factor, NF-κB, activity. Despite the utility of sulfasalazine for patients with inflammatory bowel disease, experience has shown that up to one third of patients cannot tolerate this medication and manifest one or more side effects of variable severity. These side effects are related directly to systemic absorption of sulfapyridine. Because of sulfasalazine-related side effects, investigators have examined 5-ASA as a single agent for the treatment of inflammatory bowel disease. There have been several formulations of 5-ASA designed to inhibit proximal intestinal absorption and delivery of this compound to areas of active inflammation (39). Several formulations of 5-ASA have been studied and those currently most popular include coated forms of 5-ASA that are released in a pH-sensitive manner to the distal ileum and colon. Examples of such agents include Asacol™ (Proctor and Gamble) and Pentasa™ (Shire US inc.). Furthermore, 5-ASA preparation for rectal delivery has included the development of suppositories and enemas containing 5-ASA as the active agent. Examples include Rowasa™ rectal suspension enema (Solvay Pharmaceuticals) and 5-ASA suppositories such as Canasa™ (Axcan Scandiapharm). Thus, a number of oral or rectally delivered 5-ASA agents are presently available for the treatment of mild to moderate inflammatory bowel disease.
Recent investigations into the etiological triad of genetic: environmental: immune factors have expanded our knowledge of these individual components and their potential interactions. Pathogenetic models of IBD envision initiating events, possible microbiologicals, arising from the gut lumen that converts immune tolerance to a sustained hyperactive state with elaboration and amplification of cellular and humoral mediators. Immunocyte derived injurious and proinflammatory substances cause tissue injury and destruction. These substances include prostaglandins, reactive oxygen metabolites, nitric oxide, leukotrienes, proteases and matrix metalloproteinases (3). The role of reactive oxygen species (ROS) and nitric oxide (NO) have been examined in experimental models of IBD (4, 5). Pharmacological and genetic manipulation of oxygen free radical and NO generation have been shown to ameliorate experimental colitis induced by luminal administered trinitrobenzene sulfonic acid (TNBS) and dextran sulfate sodium (DSS) (5-10).
Although several experimental strategies have been employed that suggest the importance of enhanced production of superoxide and nitric oxide in the pathogenesis of IBD, inconsistent results have the issue unresolved. For example, the beneficial effect of superoxide dismutase (SOD) treatment in experimental models of colitis has been reported while SOD treatment in humans with IBD has shown limited benefit (40). Similarly, the inhibitors of inducible nitric oxide synthase (iNOS) have yielded mixed results in various experimental models of IBD (6-8, 27). The antioxidants N-acetylcysteine (NAC) and phenyl N-tert-butylnitrone (PBN) when used alone have been shown to be effective in protection against TNBS-induced colitis in rat (9) and DSS-induced colitis in mice, (10) respectfully. Furthermore, recent studies suggest a dominant role of iNOS-derived NO in a murine model of colitis (5). Antioxidant therapy has also been shown to suppress colonic iNOS activity and to decrease colonic NF-κB DNA-binding activity in experimental animals (10). Nuclear factor-κB, NF-κB is a family of transcription factors known to regulate a variety of genes controlling the inflammatory process and regulating programmed cell death (41).
Thus, there exists extensive experimental support for the notion that reactive oxygen molecules and nitric oxide may contribute to the pathogenesis of mucosal injury in inflammatory bowel disease. Furthermore, experimental evidence also provides support for the concept that inhibition of nitric oxide species and NO generation exert favorable effects on mucosal healing and the inflammatory process in several well-defined models of chemically induced colitis. However, there continues to be a need in the field for a more effective treatment of inflammatory bowel diseases and other conditions related to inflammation. It is to this need that the present invention is directed.